Iron-base oxide dispersion strengthened alloys (iron-base ODS alloys) have been developed for high temperature applications. Chromium and aluminum are typically added to an iron-base alloy for resistance to oxidation, carburization and hot corrosion. The alloy is strengthened with an oxide stable at high temperature, such as yttrium oxide. The oxide is uniformly distributed throughout the alloy as a finely distributed dispersoid by mechanically alloying the powder. Iron-base ODS alloys in the form of sheet are particularly useful for gas-turbine combustion chambers, components of advanced energy-conversion systems and high temperature vacuum furnaces.
Generally, very coarse grains are desired in MA iron-base ODS alloys for high temperature rupture strength. The coarsening of the grains provides for increased rupture strength and decreased ductility. In sheet products, a minimum number of grains traversing the thickness may be required to provide optimal high temperature rupture strength. Typically, MA iron-base ODS alloys produced by a combination of extrusion and rolling have less than 3 to 4 grains comprising the sheet thickness. The small number of grains may cause mechanical properties to be quite variable depending on the number of grains, the orientation of the grain boundaries with respect to the axis of loading, and the orientation of the grains themselves. Variability in properties means that the designer must lower the design stresses to below that for the weakest experienced material. In addition, alloy ductility with coarse grains may also be erratic.
Properties of sheet iron-base alloys are extremely process dependent. The forming history of sheet controls ultimate strength properties produced. For high temperature rupture strength it is desired to form a coarse pancake type grain structure by performing a combination of longitudinal and cross rolling. The pancake structure provides isotropic properties in the rolling and transverse directions. Forming MA iron-base powder into sheet has required a combination of hot working operations and cold working operations. Between cold rolling operations, an intermediate temperature anneal is typically used to increase ductility. Suarez et al, is U.S. Pat. No. 5,032,190 an improved process for achieving isotropic properties in the rolling and transverse directions.
MA iron-base alloys have been formed into sheet using a multi-step process. First, iron-base alloys have been prepared by mechanical alloying metal powder components to form a suitable MA powder. MA powder was then encased in steel cladding to form a billet. The billet was then extruded at 1066.degree. C. and hot rolled at elevated temperature.
A pickling operation was then used to remove the can. To finish the sheet, the sheet was cold rolled at a temperature slightly above room temperature such as 100.degree. C. to final size. Cold working is defined as rolling at a temperature at which work hardening occurs during deformation with very little, if any, work softening or relaxation. Cold rolling at temperatures slightly above room temperature was required because iron-base ODS alloys may have a ductile to brittle transition temperature at about room temperature. Optionally, an intermediate temperature anneal at about 1090.degree. C. may be used in between a series of cold rolling operations to increase ductility. It is recognized that an intermediate temperature anneal may also affect the transition temperature. Cold working is desired to produce a sheet as close to finished gage as possible and to prevent oxide formation. However, cold working of ODS iron-base sheet has often produced sheet having less than 3 to 4 grains per thickness after a final anneal at about 1370.degree. C. This large grain size increases stress rupture strength, but it does not provide the often desired properties of decreased dependence upon grain orientation.
It is an object of this invention to provide a method for increasing control of ultimate grain size formed of annealed MA iron-base alloy.
It is an object of this invention to decrease final grain size of annealed MA iron-base alloy that has been hot worked and cold worked to finished size.
It is further an object of this invention to provide a method for decreasing grain orientation dependence and to increase sheet ductility of MA iron-base alloys.